(a) Field of the Invention
The present invention relates generally to a method for steering a satellite antenna beam or array of beams, and more particularly, to a method for simplifying the steering of an antenna beam or array of beams on a satellite in an inclined earth orbit in order to compensate for cross-track motion of earth-based terminals that is caused by rotation of the Earth.
(b) Description of Related Art
Antenna systems for communication satellites that are in non-geostationary orbits may require continuous adjustment of beam steering directions relative to the satellite to maintain coverage of users located within an earth-fixed cell during the pass of the satellite over the cell. The direction from the satellite to the users in satellite coordinates is affected by the rotation of the Earth as well as by the orbital motion of the satellite. The surface speed of the Earth due to rotation is proportional to the cosine of the latitude of the satellite, which varies throughout the orbit for all but equatorial (zero-inclination) orbits. This variation of relative velocity as a function of latitude normally requires beam steering in the cross-track direction (i.e., orthogonal to the velocity vector of the satellite) as well as in the along-track direction (i.e., along the velocity vector of the satellite), which in turn results in excessively complicated and cumbersome beam steering systems.
LEO satellite systems have the disadvantage that the satellite is in motion relative to the stationary or slowly moving user on or near the earth's surface. Users are usually grouped into cells depending on the user's geographic location. In the communications system, each cell is associated with a satellite antenna beam that transmits signals to or receives signals from the users located in a particular cell.
In prior art satellite systems, the cell-beam relationship can be described as either earth-fixed cells or satellite-fixed beams. In satellite-fixed beam systems, the beams point in fixed directions relative to the satellite body and thus sweep over the cells as the satellite moves through its orbit. As a result, the users must be reassigned to different beams frequently. There must be rapid reassignment calculations and frequent messages exchanged between the satellite and the user to coordinate the reassignment, leading to a significant overhead load being used for control (i.e., messages instead of for voice, data, video, etc.) on the system.
In earth-fixed cell systems, the satellite must continuously repoint the antenna beams to follow the motion of the cells as seen from the moving satellite. Implementing earth-fixed cells requires a very complex antenna that can steer many beams in two angular dimensions. Rapid reassignment calculations and overhead load are reduced at the expense of a vastly more complex antenna.
If the satellite antenna system is an electronically steered, high gain, low side lobe multibeam array, antenna steering may involve the control of the phase and amplitude of many elements. The number of active control elements required is substantially increased when beam steering is required in the cross-track direction as well as the along-track direction. This is normally the case, since for an antenna array aligned with the satellite geometric axes, cross-track motion results from the rotation of the Earth.